Charger and electronic apparatus stably supplying operating voltage

ABSTRACT

A charger includes a power obtaining unit and a power transfer unit. The power obtaining unit charges a first power storage device with a voltage supplied from outside. The power transfer unit transfers power stored in the first power storage device to a second power storage device so as to store the power in the second power storage device. The power transfer unit includes a voltage decrease reduction unit. The voltage decrease reduction unit reduces voltage decrease of the first power storage device per transfer of the power from the first power storage device to the second power storage device. The second power storage device has a capacitance larger than the first power storage device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority under35 USC 119 of Japanese Patent Application No. 2015-109293 filed on May29, 2015, the entire disclosure of which, including the description,claims, drawings and abstract, is incorporated herein by reference inits entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a charger and an electronic apparatusstably supplying an operating voltage.

2. Description of the Related Art

There has been an electronic apparatus usable for a long period of timewhile repeating charge and discharge which is caused by operation, byusing a power storage device such as a secondary cell. This kind ofpower storage device is (i) supplied with power (electric power) by adedicated charger, or (ii) (a) provided with a power generation meanssuch as solar power, wind power or power generated by making use ofvibration of a portable electronic apparatus and (b) charged withsurplus power of the generated power. There also has been an electronicapparatus provided with a plurality of accumulators for stable powersupply.

In this kind of electronic apparatus, conventionally, one power storagedevice is chargeable with wireless power supplied from outside. Further,there is a technology of transferring energy from this one power storagedevice to another power storage device and controlling capacitances oftheir accumulators so as to supply power to a load from at least one ofthe power storage devices (Japanese Translation of PCT InternationalApplication Publication No. 2012-530482).

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided acharger including: a power obtaining unit which charges a first powerstorage device with a voltage supplied from outside; and a powertransfer unit which transfers power stored in the first power storagedevice to a second power storage device having a capacitance larger thanthe first power storage device so as to store the power in the secondpower storage device, wherein the power transfer unit includes a voltagedecrease reduction unit which reduces voltage decrease of the firstpower storage device per transfer of the power from the first powerstorage device to the second power storage device.

BRIEF DESCRIPTION OF THE DRAWING

The present invention is fully understood from the detailed descriptiongiven hereinafter and the accompanying drawings, which are given by wayof illustration only and thus are not intended to limit the presentinvention, wherein:

FIG. 1 is a block diagram showing the overall configuration of acommunication system including an electronic apparatus including acharger;

FIG. 2 is a diagram to explain a circuit configuration of the electronicapparatus for charge and discharge according to a first embodiment ofthe present invention;

FIG. 3 shows an example of change of the state of power storage into afirst power storage device and a second power storage device of acharging circuit over time; and

FIG. 4 is a diagram to explain a circuit configuration of the electronicapparatus for charge and discharge according to a second embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention are described withreference to the drawings.

First Embodiment

First, an electronic apparatus including a charger according to a firstembodiment of the present invention is described.

FIG. 1 is a block diagram showing the overall configuration of acommunication system including an electronic apparatus including acharger according to the embodiment.

A communication system 100 includes: an electronic apparatus 10including an RF tag 12; and an external apparatus 50 which functions asan RF tag 12 reader/writer. The electronic apparatus 10 and the externalapparatus 50 perform wireless communication of Near-Field RadioCommunication (NFC) therebetween. The electronic apparatus 10 and theexternal apparatus 50 include their respective power circuits andoperate with power supplied from the respective power circuits.

The external apparatus 50 transmits radio waves for polling with anantenna 51 at predetermined time intervals in a normal state. Whendetects and receives the wireless radio waves output from the externalapparatus 50, the electronic apparatus 10 starts operating with powerobtained from variation of electromagnetic field relative to thewireless radio waves and transmits/receives communication data throughNFC, and also performs various types of processing accompanying thetransmission/reception of the communication data on the basis of powersupply from the power circuit. When detects that the electronicapparatus 10 receives the radio waves, the external apparatus 50communicates with the electronic apparatus 10 at shorter time intervalsor continuously as needed.

The electronic apparatus 10 includes a charging circuit 20 as a charger,a power circuit 11, the RF tag 12 and a microcomputer 13 (operatingunit).

The charging circuit 20 receives, with an antenna ANT, radio waves ofwireless communication from the external apparatus 50 and stores thepower, and operates the RF tag 12 and the microcomputer 13 with thestored power.

The charging circuit 20 includes the antenna ANT, a rectifier circuit21, a first power storage device 22, a switching circuit 23 (switchingunit), a current limiting circuit 24 (voltage decrease reduction unit),a second power storage device 25, a voltage detection unit 26 (transferallowing/prohibiting unit, comparison unit) and a chopper circuit 27(oscillator circuit).

The rectifier circuit 21 rectifies and smooths the received radio wavesand supplies them to the first power storage device 22 and the RF tag 12as power at the voltage value.

The antenna ANT and the rectifier circuit 21 constitute a powerobtaining unit.

The first power storage device 22 stores the power obtained from therectifier circuit 21, and supplies the power to the RF tag 12 as needed.The power stored in the first power storage device 22 is transferred tothe second power storage device 25 when the below-described condition issatisfied. The power storage amount (electrical capacitance of acapacitor as described below, simply referred to as “capacitance”) ofthe first power storage device 22 is smaller than that of the secondpower storage device 25 and set to be an amount to rapidly reach theoperating voltage of the RF tag 12 with the received power of thereceived amplitude strength of the radio waves transmitted throughwireless communication from the external apparatus 50, which is anexpected wireless-radio-wave transmitter.

The switching circuit 23 is provided at a point on a path through whichpower is transferred from the first power storage device 22 to thesecond power storage device 25, and switches connection of the path ONand OFF. This switching circuit 23 includes a switching element whichswitches the connection between a capacitor 22C (shown in FIG. 2) of thefirst power storage device 22 and a capacitor 25C (shown in FIG. 2) ofthe second power storage device 25 ON and OFF. When the switchingelement is in the ON state, power is transferred from the first powerstorage device 22 to the second power storage device 25, whereas whenthe switching element is in the OFF state, this power transfer is notperformed. Meanwhile, the power stored in the second power storagedevice 25 can be output to the power circuit 11 as needed.

The current limiting circuit 24 limits current (transfer current)flowing from the first power storage device 22 to the second powerstorage device 25 while the switching element of the switching circuit23 is in the ON state. That is, the current limiting circuit 24 preventsthe power (electric charge) stored in the first power storage device 22having a small capacitance from sharply decreasing and fluctuation(ripple) of the supply voltage (output voltage) of the first powerstorage device 22 from being large.

The second power storage device 25 stores the power transferred from thefirst power storage device 22 and supplies the power at a predeterminedvoltage to the microcomputer 13 via the power circuit 11. The secondpower storage device 25 has a capacitance sufficiently larger than thefirst power storage device 22. The one having a large capacitance isexemplified by an electric double-layer capacitor and a secondary cell.The second power storage device 25 is configured to supply power at apredetermined supply voltage being equal to or higher than the operatingvoltage of the microcomputer 13 for a preset operating time of themicrocomputer 13 or longer.

The voltage detection unit 26 detects the output voltage of the firstpower storage device 22 and determines whether or not to transfer powerto the second power storage device 25. The voltage detection unit 26detects a relationship (higher/lower relationship) between the outputvoltage of the first power storage device 22 and a high referencevoltage VH and a relationship (higher/lower relationship) between theoutput voltage of the first power storage device 22 and a low referencevoltage VL being lower than the high reference voltage VH, and outputsan operation signal to the chopper circuit 27 according to the detection(comparison) results. From when the output voltage of the first powerstorage device 22 becomes the high reference voltage VH or higher untilthe output voltage of the first power storage device 22 becomes lowerthan the low reference voltage VL, the voltage detection unit 26outputs, to the chopper circuit 27, the operation signal to allow powertransfer from the first power storage device 22 to the second powerstorage device 25 so as to make the switching element of the switchingcircuit 23 switch ON and OFF at short time intervals according to theoperation of the chopper circuit 27 and thereby repeat the powertransfer (i.e. intermittently perform the power transfer). Outside thisperiod of time, namely, (i) until the output voltage of the first powerstorage device 22 first becomes the high reference voltage VH or higher,and (ii) from when the output voltage of the first power storage device22 becomes lower than the low reference voltage VL during the powertransfer until the output voltage of the first power storage device 22becomes the high reference voltage VH or higher, the voltage detectionunit 26 outputs, to the chopper circuit 27, the operation signal toprohibit the power transfer so as to make the switching element of theswitching circuit 23 keep the OFF state.

The chopper circuit 27 performs chopping operation throughout anoperation period corresponding to a period of the operation signal (toallow the power transfer) input from the voltage detection unit 26, andthis chopping operation makes the switching element of the switchingcircuit 23 perform the ON/OFF switching operation. Further, the choppercircuit 27 outputs a signal to keep the OFF state to the switchingelement of the switching circuit 23 throughout an operation prohibitionperiod corresponding to a period of the operation signal (to prohibitthe power transfer) input from the voltage detection unit 26.

The switching circuit 23, the current limiting circuit 24 and thechopper circuit 27 constitute a power transfer unit 20 a.

The power circuit 11 receives the power stored in the second powerstorage device 25 via a backflow diode 23D (shown in FIG. 2), andoutputs the power after converting the power into a predetermined supplyvoltage at which the microcomputer 13 can operate. This supply voltagecan be set higher than the output voltage of the second power storagedevice 25. In this case, the power circuit 11 boosts the output voltageof the second power storage device 25 with a booster circuit and outputsthe boosted voltage.

The RF tag 12 is an IC chip which operates with, as power, radio wavesreceived and supplied from the external apparatus 50 via the antenna ANTand the rectifier circuit 21. The RF tag 12 includes a storage unit 121where unique identification information and predetermined statusinformation are stored, and when receives a voltage being apredetermined operating voltage or higher input from the rectifiercircuit 21, outputs a predetermined response signal to the externalapparatus 50. This response signal or transmission data to be outputafter the response signal is transmitted contains the uniqueidentification information and the status information stored in thestorage unit 121. When new status information is received and obtainedfrom the external apparatus 50, the status information stored in thestorage unit 121 is overwritten and updated with the obtained new statusinformation. The information stored in the RF tag 12 may be updated byoperation of the microcomputer 13 as needed. The RF tag 12 and themicrocomputer 13 can transmit/receive signals to/from each other via anot-shown bus.

The microcomputer 13 performs predetermined operation with the power,which is supplied from the power circuit 11 on the basis of the outputvoltage of the second power storage device 25. The operation contents ofthe microcomputer 13 are appropriately determined in advance and mayinclude operation to perform measurement with a not-shown sensor(temperature sensor or the like) at predetermined time intervals andstore the measured values in the storage unit 121 of the RF tag 12 ashistory information.

FIG. 2 shows a circuit configuration of the electronic apparatus 10according to the embodiment.

An AC voltage signal received via the antenna ANT is input to the RF tag12 as it is, and also input as a supply voltage to the RF tag 12 afterbeing rectified by the rectifier circuit 21. As the rectifier circuit21, any well-known circuit can be used, but in FIG. 2, a circuit usingone diode is shown as a simple example.

The supply voltage obtained by the rectification is also input to oneend of the capacitor 22C, which constitutes the first power storagedevice 22. The other end of the capacitor 22C is grounded, and electriccharge corresponding to the supply voltage is stored in the first powerstorage device 22.

This supply voltage, namely, the voltage at the one end of the capacitor22C (output voltage of the capacitor 22C), is also input to the powercircuit 11, the voltage detection unit 26 and the switching circuit 23.

The voltage detection unit 26 includes two voltage detectors Cp1 and Cp2and a controller 261. The voltage detector Cp1 compares the highreference voltage VH with the supply voltage and outputs the comparisonresult to the controller 261. The voltage detector Cp2 compares the lowreference voltage VL with the supply voltage and outputs the comparisonresult to the controller 261. From when the supply voltage becomes lowerthan the low reference voltage VL until the supply voltage becomes thehigh reference voltage VH (upper limit voltage) or higher, thecontroller 261 outputs a low level signal to prohibit the choppingoperation to the chopper circuit 27, and from when the supply voltagebecomes the high reference voltage VH or higher until the supply voltagebecomes lower than the low reference voltage VL, the controller 261outputs a high level signal to allow the chopping operation to thechopper circuit 27. That is, the voltage detection unit 26 allows thetransfer from the first power storage device 22 to the second powerstorage device 25 while keeping the charged voltage of the first powerstorage device 22 from falling below the low reference voltage VL(reference voltage). The controller 261 is formed of an IC chip having alogic circuit which can output the above-described signals, such as aflip-flop circuit. Setting the low reference voltage VL to the operatingvoltage of the RF tag 12 or higher can prevent the once-started RF tag12 from going down.

The voltage detectors Cp1 and Cp2 output the ground voltage when detectthat voltage lower than their respective reference voltages is input,and output voltage as it is when detect that voltage equal to or higherthan their respective reference voltages is input.

The chopper circuit 27 compares (i) the output signal from thecontroller 261 according to the comparison results of the voltagedetectors Cp1 and Cp2 with (ii) a signal (“comparison signal” below)obtained by subjecting the output signal of the chopper circuit 27 to alow-pass filter composed of a resistor and a capacitor connected inseries, and outputs the comparison result to a transistor (FET 23T),which is the switching element of the switching circuit 23. The outputsignal of the chopper circuit 27 has two values, a high level and a lowlevel. The low level output signal has a lower voltage than the lowlevel signal output from the controller 261, and the high level outputsignal has a higher voltage than the high level signal output from thecontroller 261. Thereby, during input of the high level signal from thecontroller 261, (i) when time corresponding to time constant determinedby the resistance value of the resistor and the capacitance of thecapacitor elapses after output of the comparator changes from the lowlevel to the high level, the comparison signal becomes the voltage ofthe high level signal output from the controller 261 or higher and theoutput of the comparator changes to the low level, and (ii) when timecorresponding to the time constant elapses after output of thecomparator changes from the high level to the low level, the comparisonsignal becomes lower than the voltage of the high level signal outputfrom the controller 261 and the output of the comparator changes to thehigh level. That is, the chopper circuit 27 constitutes an oscillatorcircuit, and the FET 23T of the switching circuit 23 repeatedly switchesON and OFF according to the oscillation operation of the oscillatorcircuit.

The switching circuit 23 includes the FET 23T and the backflow diode23D.

The gate terminal of the FET 23T is connected to the output of thechopper circuit 27, the source terminal thereof is connected to the oneend of the capacitor 22C of the first power storage device 22, and thedrain terminal thereof is connected to the current limiting circuit 24.The FET 23T switches ON and OFF according to the level (high level/lowlevel) of the output signal of the chopper circuit 27. Throughout ON ofthe FET 23T, the FET 23T transfers the power stored in the capacitor 22Cof the first power storage device 22 to the capacitor 25C of the secondpower storage device 25.

Throughout OFF of the FET 23T, the backflow diode 23D prevents the powerfrom transferring from the first power storage device 22 to the secondpower storage device 25, but can output the power stored in the secondpower storage device 25 to the power circuit 11 when the microcomputer13 operates.

The current limiting circuit 24 includes an inductor 24L and a diode24D. One end of the inductor 24L is connected to the drain terminal ofthe FET 23T and the cathode of the diode 24D, and the other end thereofis connected to one end of the capacitor 25C. That is, the inductor 24Lis provided at a point on the path where the transfer current flows fromthe capacitor 22C to the capacitor 25C. The anode of the diode 24D isgrounded.

This prevents sharp increase in the current flowing from the capacitor22C to the capacitor 25C in the ON state of the FET 23T. Further, theperiod throughout which the FET 23T stays ON according to the output ofthe chopper circuit 27 is properly set to a predetermined periodaccording to the inductance of the inductor 24L or shorter. Thisprevents the current from the capacitor 22C to the capacitor 25C frombeing large current. Meanwhile, the induced current which is generatedin the inductor 24L flows through the diode 24D immediately after theFET 23T becomes OFF. This compensates charging of the capacitor 25C.

The second power storage device 25 includes the capacitor 25C. One endof the capacitor 25C is connected to the inductor 24L, and the other endthereof is grounded. The capacitor 25C has a sufficiently largercapacitance than the capacitor 22C, and, as described above, as thecapacitor 25C, an electric double-layer capacitor is used, for example.

FIG. 3 shows an example of change of the state of power storage into thefirst power storage device 22 and the second power storage device 25 ofthe charging circuit 20 over time according to the embodiment.

First, in the state in which the voltage V_(C22) of the capacitor 22C ofthe first power storage device 22 is lower than the low referencevoltage VL, the chopper circuit 27 stops operating in response to theoperation signal output from the controller 261, whereby the switchingcircuit 23 becomes and keeps the OFF state (a transferallowing/prohibiting step). Thereafter, even when the voltage V_(C22) ofthe capacitor 22C becomes the low reference voltage VL or higher, theswitching circuit 23 keeps the OFF state and the capacitor 22C ischarged (timings (t1) to (t2), a power obtaining step) until the voltageV_(C22) becomes the high reference voltage VH or higher. Because thecapacitance of the capacitor 22C is small, the voltage V_(C22) of thecapacitor 22C rapidly increases.

When the voltage V_(C22) of the capacitor 22C becomes the low referencevoltage VL or higher and further becomes the high reference voltage VHor higher, the chopper circuit 27 becomes capable of operating inresponse to the operation signal output from the controller 261 (thetransfer allowing/prohibiting step) and starts oscillation operation,whereby the switching circuit 23 repeatedly performs the ON/OF switchingoperation at short time intervals (timings (t2) to (t3)). FIG. 3schematically shows the state in which Transfer ON and Transfer OFF arealternated. Thereby, power is intermittently transferred from the firstpower storage device 22 to the second power storage device 25 (a powertransfer step). During the time, thanks to the operation of the currentlimiting circuit 24, the current flowing between the capacitor 22C ofthe first power storage device 22 and the capacitor 25C of the secondpower storage device 25 does not accompany loss by heat generation,which is caused by a resistor(s) or the like, and therefore does notbecome short-circuit current. Further, this slows speed of voltagedecrease of the capacitor 22C caused by individual transfer currentsduring the chopping operation, and reduces voltage decrease per transfertime (per transfer current) during the chopping operation. (In FIG. 3,the width (in the vertical direction) of the hatched part representing afalling period in a line showing change of the voltage V_(C22) issmall.)

This power transfer by the chopping operation decreases the voltageV_(C22) of the capacitor 22C by degrees and increases the voltageV_(C25) of the capacitor 25C. Because the capacitance of the capacitor25C is much larger than that of the capacitor 22C, voltage increase ofthe capacitor 25C is more gradual than voltage decrease of the capacitor22C.

When the voltage V_(C22) of the capacitor 22C falls below the lowreference voltage VL, the operation signal from the controller 261changes, whereby the chopper circuit 27 stops operating (timings (t3) to(t4)). Thereby, the switching circuit 23 keeps the transfer current inthe OFF state, and only the capacitor 22C is charged until the voltageV_(C22) of the capacitor 22C becomes the high reference voltage VH orhigher again.

As described above, the charging circuit 20 provided in the electronicapparatus 10 of the embodiment includes: the antenna ANT and therectifier circuit 21 as the power obtaining unit which charges the firstpower storage device 22 (capacitor 22C) with a voltage supplied from theexternal apparatus 50; and the power transfer unit 20 a which transferspower stored in the first power storage device 22 to the second powerstorage device 25 having a capacitance larger than the first powerstorage device 22 so as to store the power in the second power storagedevice 25. The power transfer unit 20 a includes the current limitingcircuit 24 which reduces voltage decrease of the first power storagedevice 22 per transfer of the power from the first power storage device22 to the second power storage device 25.

This can rapidly increase the output voltage of the first power storagedevice 22 to the operating voltage of the RF tag 12, and also canproperly transfer the power stored in the first power storage device 22to the second power storage device 25 while preventing the ripple of theoutput voltage from being large.

The power transfer unit 20 a performs chopping operation to perform thetransfer intermittently. This can reduce the power transfer amount pertransfer of the power, thereby reducing voltage decrease of the firstpower storage device 22, while minutely and rapidly charging the secondpower storage device 25.

The current limiting circuit 24 includes the inductor 24L provided at apoint on the path of the transfer current flowing between the firstpower storage device 22 and the second power storage device 25. Duringthe chopping operation, the inductor 24L (i) reduces the transfercurrent while the transfer is being performed, thereby reducing voltagedecrease of the first power storage device 22 (capacitor 22C), and (ii)generates current and stores power in the second power storage device 25(capacitor 25C) after the transfer is suspended. This can prevent thetransfer current from short-circuiting and hence can more easily andcertainly reduce voltage decrease of the first power storage device 22,and also can let the current flow in the inductor 24L after the transferis suspended and hence can compensate the reduced/suspended power supplywith the current and accordingly more efficiently charge the electronicapparatus 10.

The power transfer unit 20 a includes: the chopper circuit 27, whichconstitutes the oscillator circuit; and the switching circuit 23repeatedly switches connection between the first power storage device 22and the second power storage device 25 on and off according to theoscillation operation of the oscillator circuit, with the FET 23Tprovided at a point on the path of the transfer current between thefirst power storage device 22 and the second power storage device 25.

Thereby, the chopping operation can be easily and stably performed, andalso the power transfer can be performed within a proper range and hencethe ripple accompanying the voltage decrease of the first power storagedevice 22 can be prevented from being large.

The charging circuit 20 includes the voltage detection unit 26 as thetransfer allowing/prohibiting unit which (i) does not allow the transferwhile the output voltage of the first power storage device 22 is lowerthan the predetermined low reference voltage VL, and (ii) allows thepower transfer unit 20 a to transfer power from the first power storagedevice 22 to the second power storage device 25, namely, allows thetransfer, from when the output voltage of the first power storage device22 becomes the predetermined high reference voltage VH or higher, thehigh reference voltage VH being higher than the low reference voltageVL, until the output voltage of the first power storage device 22becomes lower than the low reference voltage VL. This can keep theoutput voltage of the first power storage device 22 at a proper levelwhich is the operating voltage of the RF tag 12 or higher while easilyand efficiently transferring the power to the second power storagedevice 25.

The voltage detection unit 26 includes: the voltage detectors Cp1 andCp2 as the comparison unit which compares the output voltage of thefirst power storage device 22 with each of the low reference voltage VLand the high reference voltage VH; and the controller 261 determineswhether or not to allow the power transfer unit 20 a to perform thetransfer on the basis of the comparison result of the comparison unit.Thereby, whether or not to allow the power transfer can be easily andproperly determined on the basis of the two end voltages (the lowreference voltage VL and the high reference voltage VH) and thehigher/lower relationship between the output voltage of the first powerstorage device 22 and each of the reference voltages only, withoutcaring any voltage between the two end voltages.

The voltage detection unit 26 stops the oscillation operation of thechopper circuit 27 while not allowing the power transfer unit 20 a toperform the transfer, and the switching circuit 23 keeps, with the FET23T, the connection of the path, through which the power is transferred,between the first power storage device 22 and the second power storagedevice 25 off while the oscillation operation of the chopper circuit 27stops. This makes it easy to control the switching operation of the FET23T.

The power obtaining unit includes: the antenna ANT which receiveswireless radio waves transmitted from the external apparatus 50; and therectifier circuit 21 which rectifies current of the received wirelessradio waves. This makes it possible to use the same antenna for bothreceiving wireless radio waves, which is ordinary operation, andobtaining power and hence can reduce circuit size and simplify a circuitdue to the received radio waves used for obtaining power.

The antenna ANT receives the wireless radio waves transmitted throughNear-Field Radio Communication (NFC) from the external apparatus 50which is involved in at least one of reading and writing of the RF tag12. Hence, the present invention can be effectively applied, inparticular, to an electronic apparatus including a device which canoperate without preparing power by itself, such as the RF tag 12.

The capacitor 25C of the second power storage device 25 is one of asecondary cell and an electric double-layer capacitor, and hence canstably store a large amount of electricity (power).

The first power storage device 22 is the capacitor 22C, and hence caneasily store a small amount of electricity (power) with a small size.

The electronic apparatus 10 of the embodiment includes, in addition tothe charging circuit 20, the RF tag 12 which operates with, as power,wireless radio waves transmitted from the external apparatus 50 throughNear-Field Radio Communication; and the microcomputer 13 which performspredetermined operation with the power stored in the second powerstorage device 25. Hence, when the RF tag 12 is operated, and themicrocomputer 13 is operated relative or irrelative to the operation ofthe RF tag 12, the second power storage device 25 is charged accordingto the operation of the RF tag 12, so that the microcomputer 13 canoperate. This can save time and effort to prepare a battery for themicrocomputer 13 or to separately charge the microcomputer 13. Further,when the RF tag 12 performs active transmission or the like inparticular, the second power storage device 25 can be effectivelycharged according to the operation of the RF tag 12. This enablesefficient and comfortable wireless communication with the RF tag 12.

The low reference voltage VL is determined to be higher than theoperating voltage of the RF tag 12. This enables continuous supply ofthe operating voltage of the RF tag 12 to the RF tag 12 and hence canstably operate the RF tag 12 while storing the power into the secondpower storage device 25. In particular, in the case where the frequencyof communication received for polling in the waiting state in which theRF tag 12 does not operate is lower than the frequency of communicationin the ordinary continuous operation state in which the RF tag 12 is inoperation, obtainable power per unit time in the waiting state is low,and hence efficient charging cannot be performed. Then, keepingcontinuous supply of the operating voltage enables more rapid charging.

Second Embodiment

Next, the electronic apparatus 10 including a charger according to asecond embodiment of the present invention is described.

FIG. 4 shows an example of a circuit configuration of the electronicapparatus 10 for charge and discharge according to the secondembodiment, wherein the high reference voltage VH and the low referencevoltage VL are variable.

The voltage obtained by rectifying, with the rectifier circuit 21, theradio waves received by the antennal ANT is supplied to the capacitor22C of the first power storage device 22 and also sampled by a sampleand hold circuit 262 at appropriate timing, whereby a voltage value V isheld. This held voltage value V is transformed at an upper sidetransformer 263 by adding a difference value ΔV (a predetermined valueproper to determine the difference between the high reference voltage VHand the low reference voltage VL) to a half value V/2 of the voltagevalue V and then input to a voltage detector Cp1 a as the high referencevoltage VH or as a value to set the high reference voltage VH, and alsotransformed at a lower side transformer 264 by subtracting thedifference value ΔV from the half value V/2 and then input to a voltagedetector Cp2 a as the low reference voltage VL or as a value to set thelow reference voltage VL.

Charging efficiency of the second power storage device 25 is increasedby thus determining the high reference voltage VH and the low referencevoltage VL, which are compared with the supply voltage (output voltage)of the first power storage device 22, such that input of about half ofthe open circuit voltage of the antenna ANT operates the controller 261to charge the capacitor 25C of the second power storage device 25.

As described above, in the charging circuit 20 of the electronicapparatus 10 of the second embodiment of the present invention, the lowreference voltage VL can be determined according to the amplitude of theinput voltage via the antenna ANT, namely, according to the outputvoltage of the first power storage device 22, thereby being variable.

In addition to the low reference voltage VL, the upper reference voltageVH can also be determined according to the output voltage of the firstpower storage device 22, thereby being variable.

This enables the power transfer unit 20 a to transfer the power within avoltage range proper in terms of, for example, power transferefficiency.

The present invention is not limited to the above embodiments and can bemodified in a variety of aspects.

For example, in the above embodiments, charging is performed byreceiving NFC radio waves as wireless radio waves. However, the wirelesscommunication is not limited to NFC. The forms relative to signaltransmission, such as the frequency, modulation system and signaltransmission format, are appropriately determined. Charging may beperformed, other than through wireless communication, by obtainingelectromagnetic waves transmitted not for communication, variation ofelectromagnetic field in space generated by electromagnetic inductionwith the inductor component of an electrical circuit, or AC voltagesupplied through a cable.

Further, in the above embodiments, the first power storage device 22includes the ordinary capacitor 22C, and the second power storage device25 includes the capacitor 25C of an electric double-layer capacitor orthe like. These are not limitations. For example, as the second powerstorage device 25, a secondary cell may be used. Further, the componentin the first power storage device 22 and/or the second power storagedevice 25 is not limited to a single component, and thus may be two ormore components connected in parallel.

Further, in the above embodiments, in terms of reduction in power lossand charging efficiency in OFF of the FET 23T, the inductor 24L is usedfor slowing the speed of voltage decrease. Additionally oralternatively, a resistor having a low resistance may be used. Furtheradditionally or alternatively, a third power storage device having asmall or medium capacitance may be provided between the first powerstorage device 22 and the second power storage device 25 so as not todrastically reduce potential difference between the first power storagedevice 22 and the second power storage device 25.

Further, in the above embodiments, the chopping operation is performedat a fixed frequency corresponding to that of the oscillation operationof the chopper circuit 27. However, the chopper circuit 27 may beconfigured to make the fixed frequency variable, or a component tochange the duty cycle according to, for example, potential differencebetween the first power storage device 22 and the second power storagedevice 25 may be provided.

Further, in the above embodiments, from when the output voltage of thecapacitor 22C becomes the high reference voltage VH or higher until theoutput voltage thereof becomes lower than the low reference voltage VL,the chopping operation is performed to transfer power. However, this isnot at all intended to prevent the transfer operation from when theoutput voltage of the capacitor 22C becomes the low reference voltage VLor higher until the output voltage thereof becomes the high referencevoltage VH or higher. Depending on the situation, the power transfer maybe performed during this period at a duty cycle lower than that of thepower transfer in the above embodiments.

Further, in the above embodiments, two voltage detectors, Cp1 and Cp2 orthe like, are used to detect the higher/lower relationship between theoutput voltage of the first power storage device 22 and each of the highreference voltage VH and the low reference voltage VL, and thecontroller 261 is used to determine whether or not to operate thechopper circuit 27. Detection of the relationship between the outputvoltage of the first power storage device 22 and each of these referencevoltages, determination of whether or not to operate the chopper circuit27 and the like may be performed by other component (s).

Further, in the above embodiments, the electronic apparatus 10 includesthe RF tag 12 which operates with wireless radio waves. However, it ispossible that the electronic apparatus 10 does not include the RF tag12, and wireless radio waves are simply used as a trigger for startingthe microcomputer 13. In this case, the low reference voltage VL may bedetermined according to not the operating voltage of the RF tag 12 butdetection accuracy of the start trigger or the like. The electronicapparatus 10 may include, instead of the RF tag 12, another componentwhich performs operation.

In addition to the above, the specific details described in the aboveembodiments, such as circuit configurations and operation contents, canbe appropriately modified within a scope not departing from the spiritof the present invention.

In the above, several embodiments of the present invention aredescribed. However, the scope of the present invention is not limited tothe above embodiments but includes the scope of the present inventionstated in claims below and the scope of equivalents.

What is claimed is:
 1. A charger comprising: a power obtaining unitwhich charges a first power storage device with a voltage supplied fromoutside; and a power transfer unit which transfers power stored in thefirst power storage device to a second power storage device having acapacitance larger than the first power storage device so as to storethe power in the second power storage device, wherein the powerobtaining unit includes: an antenna which receives a wireless radio wavetransmitted from an external apparatus; and a rectifier circuit whichrectifies current of the received wireless radio wave, and wherein thepower transfer unit includes a voltage decrease reduction unit whichreduces voltage decrease of the first power storage device per transferof the power from the first power storage device to the second powerstorage device.
 2. The charger according to claim 1, wherein the powertransfer unit performs a chopping operation to perform the transferintermittently.
 3. The charger according to claim 2, wherein: thevoltage decrease reduction unit includes an inductor provided at a pointon a path of transfer current flowing between the first power storagedevice and the second power storage device, and during the choppingoperation, the inductor (i) reduces the transfer current while thetransfer is being performed, thereby reducing the voltage decrease ofthe first power storage device, and (ii) generates current and storespower in the second power storage device after the transfer issuspended.
 4. The charger according to claim 1, further comprising atransfer allowing or prohibiting unit which (i) does not allow thetransfer while an output voltage of the first power storage device islower than a reference voltage, and (ii) allows the transfer from whenthe output voltage of the first power storage device becomes an upperlimit voltage or higher, the upper limit voltage being higher than thereference voltage, until the output voltage of the first power storagedevice becomes lower than the reference voltage.
 5. The chargeraccording to claim 4, wherein the transfer allowing or prohibiting unit:includes a comparison unit which compares the output voltage of thefirst power storage device with each of the reference voltage and theupper limit voltage, and determines whether or not to allow the transferbased on a comparison result of the comparison unit.
 6. The chargeraccording to claim 4, wherein: the power transfer unit includes: anoscillator circuit; and a switching unit which repeatedly switchesconnection between the first power storage device and the second powerstorage device on and off according to oscillation operation of theoscillator circuit, the transfer allowing or prohibiting unit stops theoscillation operation of the oscillator circuit while not allowing thetransfer, and the switching unit keeps the connection off while theoscillation operation of the oscillator circuit stops.
 7. The chargeraccording to claim 1, wherein the antenna receives the wireless radiowave transmitted through near-field radio communication from theexternal apparatus which is involved in at least one of reading andwriting of an RF tag.
 8. The charger according to claim 1, wherein thesecond power storage device is one of a secondary cell and an electricdouble-layer capacitor.
 9. The charger according to claim 1, wherein thefirst power storage device is a capacitor.
 10. The charger according toclaim 4, wherein the reference voltage is determined according to theoutput voltage, thereby being variable.
 11. The charger according toclaim 4, wherein the reference voltage and the upper limit voltage aredetermined according to the output voltage, thereby being variable. 12.A charger comprising: a power obtaining unit which charges a first powerstorage device with a voltage supplied from outside: and a powertransfer unit which transfers power stored in the first power storagedevice to a second power storage device having a capacitance larger thanthe first power storage device so as to store the power in the secondpower storage device, by performing a chopping operation to perform thetransfer intermittently, wherein the power transfer unit includes: anoscillator circuit; a switching unit which repeatedly switchesconnection between the first power storage device and the second powerstorage device on and off according to oscillation operation of theoscillator circuit; and a voltage decrease reduction unit which reducesvoltage decrease of the first power storage device per transfer of thepower from the first power storage device to the second power storagedevice according to the switching unit switching the connection betweenthe first power storage device and the second power storage device onand off.
 13. An electronic apparatus comprising: the charger accordingto claim 1; an RF tag which operates with, as power, the wireless radiowave, which is transmitted from the outside through near-field radiocommunication; and an operating unit which performs an operation withthe power stored in the second power storage device.
 14. The electronicapparatus according to claim 13, wherein: the charger further comprisesa transfer allowing or prohibiting unit which (i) does not allow thetransfer while an output voltage of the first power storage device islower than a reference voltage, and (ii) allows the transfer from whenthe output voltage of the first power storage device becomes an upperlimit voltage or higher, the upper limit voltage being higher than thereference voltage, until the output voltage of the first power storagedevice becomes lower than the reference voltage, and the referencevoltage is determined to be higher than an operating voltage of the RFtag.
 15. A charging method of a charger which obtains a voltage suppliedfrom an external apparatus the charger comprising an antenna whichreceives a wireless radio wave transmitted from the external apparatus,and a rectifier circuit which rectifies current of the received wirelessradio wave, the method comprising: a power obtaining step of charging afirst power storage device with a voltage supplied from the externalapparatus; and a power transfer step of transferring power stored in thefirst power storage device to a second power storage device having acapacitance larger than the first power storage device so as to storethe power in the second power storage device, wherein the powerobtaining step includes: a receiving step of receiving, via the antenna,the wireless radio wave transmitted from the external apparatus; and arectifying step of rectifying, by the rectifier circuit, current of thereceived wireless radio wave, and wherein the power transfer stepincludes a voltage decrease reduction step of reducing voltage decreaseof the first power storage device per transfer of the power from thefirst power storage device to the second power storage device.
 16. Thecharging method according to claim 15, wherein in the power transferstep, a chopping operation to perform the transfer intermittently isperformed.
 17. The charging method according to claim 15, furthercomprising a transfer allowing/prohibiting step of (i) not allowing thetransfer while an output voltage of the first power storage device islower than a reference voltage, and (ii) allowing the transfer from whenthe output voltage of the first power storage device becomes an upperlimit voltage or higher, the upper limit voltage being higher than thereference voltage, until the output voltage of the first power storagedevice becomes lower than the reference voltage.
 18. The charging methodaccording to claim 17, wherein: the transfer allowing or prohibitingstep includes a comparison step of comparing the output voltage of thefirst power storage device with each of the reference voltage and theupper limit voltage, and in the transfer allowing or prohibiting,whether or not to allow the transfer is determined based on a comparisonresult of the comparison step.